Why does Marlow consider it lucky that "the inner truth is hidden"?

Marlow is weary of the lies and deceit of the Pilgrims. He would like to tell the Pilgrims how he feels about them, but thinks it is best to holdback,

“When you have to attend to things of that sort, to the mere incidents of the surface, the reality – the reality, I tell you – fades. The inner truth is hidden – luckily, hidden. But I felt it all the same.”